H2 is a

relational database management system A relational database is a (most commonly digital) database based on the relational model of data, as proposed by E. F. Codd in 1970. A system used to maintain relational databases is a relational database management system (RDBMS). Many relatio ...

written in

Java Java (; id, Jawa, ; jv, ꦗꦮ; su, ) is one of the Greater Sunda Islands in Indonesia. It is bordered by the Indian Ocean to the south and the Java Sea to the north. With a population of 151.6 million people, Java is the world's List ...

. It can be embedded in Java applications or run in client-server mode. The software is available as

open source Open source is source code that is made freely available for possible modification and redistribution. Products include permission to use the source code, design documents, or content of the product. The open-source model is a decentralized sof ...

software

Mozilla Public License The Mozilla Public License (MPL) is a free and open-source weak copyleft license for most Mozilla Foundation software such as Firefox and Thunderbird The MPL license is developed and maintained by Mozilla, which seeks to balance the concerns ...

2.0 or the original

Eclipse Public License The Eclipse Public License (EPL) is a free and open source software license most notably used for the Eclipse IDE and other projects by the Eclipse Foundation. It replaces the Common Public License (CPL) and removes certain terms relating to ...

History

The development of the H2 database engine started in May 2004, and first published in December 2005. The database engine was written by Thomas Mueller. He also developed the Java database engine Hypersonic SQL. In 2001, the Hypersonic SQL project was stopped, and the

HSQLDB HSQLDB (''Hyper SQL Database'') is a relational database management system written in Java. It has a JDBC driver and supports a large subset of SQL-92, SQL:2008, SQL:2011, and SQL:2016 standards. It offers a fast, small (around 1300 kilobyte ...

Group was formed to continue work on the Hypersonic SQL code. The name H2 stands for Hypersonic 2, however H2 Does not share code with Hypersonic SQL or HSQLDB. H2 is built from scratch.

Main features

Use of SQL

A subset of the SQL (Structured Query Language) standard is supported. The main programming APIs are SQL and

JDBC Java Database Connectivity (JDBC) is an application programming interface (API) for the programming language Java, which defines how a client may access a database. It is a Java-based data access technology used for Java database connectivity. I ...

, however the database also supports using the PostgreSQL ODBC driver by acting like a PostgreSQL server.

Table types

It is possible to create both in-memory tables, as well as disk-based tables. Tables can be persistent or temporary. Index types are hash table and tree for in-memory tables, and

b-tree In computer science, a B-tree is a self-balancing tree data structure that maintains sorted data and allows searches, sequential access, insertions, and deletions in logarithmic time. The B-tree generalizes the binary search tree, allowing for ...

for disk-based tables. All data manipulation operations are transactional. Table level locking and

multiversion concurrency control Multiversion concurrency control (MCC or MVCC), is a concurrency control method commonly used by database management systems to provide concurrent access to the database and in programming languages to implement transactional memory. Description ...

are implemented. The 2-phase commit protocol is supported as well, but no standard API for distributed transactions is implemented.

Security features

The security features of the database are: role based access rights, encryption of the password using

SHA-256 SHA-2 (Secure Hash Algorithm 2) is a set of cryptographic hash functions designed by the United States National Security Agency (NSA) and first published in 2001. They are built using the Merkle–Damgård construction, from a one-way compressi ...

and data using the AES or the Tiny Encryption Algorithm, XTEA. The cryptographic features are available as functions inside the database as well. SSL / TLS connections are supported in the client-server mode, as well as when using the console application. The database supports protection against

SQL injection In computing, SQL injection is a code injection technique used to attack data-driven applications, in which malicious SQL statements are inserted into an entry field for execution (e.g. to dump the database contents to the attacker). SQL in ...

by enforcing the use of parameterized statements. In H2, this feature is called 'disabling literals'.

Full text search capability

Two

full text search In text retrieval, full-text search refers to techniques for searching a single computer-stored document or a collection in a full-text database. Full-text search is distinguished from searches based on metadata or on parts of the original texts ...

implementations are included, a native implementation and one using

Lucene Apache Lucene is a free and open-source search engine software library, originally written in Java by Doug Cutting. It is supported by the Apache Software Foundation and is released under the Apache Software License. Lucene is widely used as ...

High availability

A simple form of high availability is implemented: when used in the client-server mode, the database engine supports hot failover (this is commonly known as clustering). However, the clustering mode must be enabled manually after a failure.

Cloud version

Since version 1.1.111, H2

in-memory database An in-memory database (IMDB, or main memory database system (MMDB) or memory resident database) is a database management system that primarily relies on main memory for computer data storage. It is contrasted with database management systems that e ...

can run inside the

Google App Engine Google App Engine (often referred to as GAE or simply App Engine) is a cloud computing platform as a service for developing and hosting web applications in Google-managed data centers. Applications are sandboxed and run across multiple server ...

Challenges affecting durability of relational databases

The H2 documentation explains in detail several ways in which problems in underlying hardware and in particular power systems can impact durability of relational databases. According to the H2 documentation, such problems are related not only to the DB engine design, but also to the storage caching mechanism. As storage devices use write cache in order to increase speed, in a situation of power failure, data in the device's cache is lost. Administrators have then to assess the common trade off between speed and data loss risks in the context of the business requirements and must carefully consider the design of the power supply and UPS of critical servers. It is possible in some cases to force the storage to write cache frequently or even immediately using fsync which slows the writing process or one can accept that caching in the device buffer introduces some risk of data loss in case of power failure. The effectiveness of using fsync is limited by the fact that many HDD have

write caching Writing is a medium of human communication which involves the representation of a language through a system of physically inscribed, mechanically transferred, or digitally represented symbols. Writing systems do not themselves constitute ...

enabled by factory default in which case there is nothing about the design or settings of the database nor OS level commands that will be able to eliminate the chance of lost or inconsistent data in the event of a sudden power failure. Working with the OS and hardware settings to disable all caching so as to write data in real time can have significant impacts on performance in that only around 100 write operations per second would be achievable when using spinning disks. The H2 documentation makes an effort to describe in detail potential problems with

durability Durability is the ability of a physical product to remain functional, without requiring excessive maintenance or repair, when faced with the challenges of normal operation over its design lifetime. There are several measures of durability in us ...

(part of

ACID In computer science, ACID ( atomicity, consistency, isolation, durability) is a set of properties of database transactions intended to guarantee data validity despite errors, power failures, and other mishaps. In the context of databases, a sequ ...

) resulting from the potential

data loss Data loss is an error condition in information systems in which information is destroyed by failures (like failed spindle motors or head crashes on hard drives) or neglect (like mishandling, careless handling or storage under unsuitable conditions) ...

of committed transactions in case of a

power failure A power outage (also called a powercut, a power out, a power failure, a power blackout, a power loss, or a blackout) is the loss of the electrical power network supply to an end user. There are many causes of power failures in an electricity ...

. Considering the hardware limitations regarding preservation of data in the event of sudden power loss and the ineffectiveness of approaches commonly employed by developers to prevent these sorts of losses, many database engines do not by default call FileDescriptor.sync() nor FileChannel.force() nor fsync or equivalents for every commit because they significantly degrade system performance without significantly increasing durability. HSQLDB documentation references workarounds to a similar set of issues in their documentation. The Microsoft Knowledge Base describes the impact these issues, power failures, write caching, etc. can have on performance and durability. The knowledge base discusses the trade offs between performance and the vulnerability of disk write caching as well as settings that an administrator can use to balance these.

Utilities

An embedded

web server A web server is computer software and underlying hardware that accepts requests via HTTP (the network protocol created to distribute web content) or its secure variant HTTPS. A user agent, commonly a web browser or web crawler, initiate ...

with a browser based console application is included, as well as command line tools to start and stop a server, backup and restore databases, and a command line shell tool.

References

External links

* {{Official website, www.h2database.com
H2 database tutorial
Cross-platform free software Embedded databases Free computer libraries Free database management systems Java (programming language) libraries Relational database management software for Linux Relational database management systems Serverless database management systems Software using the Mozilla license